Allow overlapping Zones if binding addresses are different (#1530)

* add OverlapChecker, move the test of overlap AFTER the directive setup process, change key of configs to allow multiple same key

* glitch when rebase. init of Config should include the default host

* add tests for the registering of configuration
rename multicast in 'unbound'.
add comments on the validator

* - merged zoneAddr and addrKey that are very similar
- move maps of Validator to zoneAddr, avoinding need to have string representation of zoneaddr
- moving key build for saving Config at Config side instead of dnsContext

* - UT on saving config is now useless.

* - cannot cleanup access to Configs after setup. Deferred function to Start, use it

* - cleanup register unit tests. remove useless function

* - address comments of review. name of validator, comments, simplify registerAndCheck

* - fixes after review. renaming a function and a comment

core/dnsserver/server.go

@@ -65,6 +65,10 @@ func NewServer(addr string, group []*Config) (*Server, error) {
 		// set the config per zone
 		s.zones[site.Zone] = site
 		// compile custom plugin for everything
+		if site.registry != nil {
+			// this config is already computed with the chain of plugin
+			continue
+		}
 		var stack plugin.Handler
 		for i := len(site.Plugin) - 1; i >= 0; i-- {
 			stack = site.Plugin[i](stack)